1. Field of the Invention
This invention relates to polymer electrolyte membrane fuel cells and methods for producing components thereof. More particularly, this invention relates to a method for producing gas diffusion electrodes and membrane electrode assemblies for polymer electrolyte membrane fuel cells.
2. Description of Prior Art
A polymer electrolyte membrane fuel cell is an electrochemical device comprising an anode electrode, a cathode electrode and an electrolyte in the form of a thin polymeric membrane disposed between the anode electrode and the cathode electrode. Individual polymer electrolyte membrane fuel cells or fuel cell units are stacked with bipolar separator plates separating the anode electrode of one fuel cell unit from the cathode electrode of an adjacent fuel cell unit to produce polymer electrolyte membrane fuel cell stacks. Conventionally, the electrodes are gas diffusion electrodes that are bonded or applied on either side of the solid polymer electrolyte membrane to produce a membrane/electrode assembly (MEA).
The gas diffusion electrode is a porous, electron-conductive layer that is disposed between a catalyst layer and the bipolar separator plates (current collectors). The porous nature of the material comprising the electrode ensures effective diffusion of each reactant gas to the catalyst on the membrane/electrode assembly. In addition, the porous nature of the material also assists in water management during operation of the fuel cell. Too little water causes a high internal resistance due to low humidification of the polymeric membrane while too much water causes flooding of the fuel cell by the water.
A variety of methods for producing gas diffusion electrodes are known including filtration, powder vacuum deposition, spray deposition, electrodeposition, casting, extrusion, and rolling and printing. However, some of these methods are very difficult to scale up to fabricate gas diffusion electrodes with good surface conductivity, gas permeability, uniformity, and long-term hydrophobic and hydrophilic stability.
U.S. Pat. No. 5,998,057 teaches a porous gas diffusion electrode for polymer electrolyte membrane fuel cells which is produced by impregnating a carbonized fiber nonwoven fabric with a mixture of soot suspension and polytetrafluoroethylene suspension, drying the impregnated material at elevated temperatures followed by sintering. A catalytically active layer comprising a noble metal catalyst on a carbon carrier mixed with an ion-conducting polymer in solution or suspension is applied to the sintered fabric. The gas diffusion electrode is combined with a polymer electrolyte membrane so as to form an MEA by pressing the electrode onto the membrane so as to provide contact between the membrane and the catalytically active layer.
U.S. Pat. No. 5,783,325 teaches a method for preparation of gas diffusion electrodes for use in solid polymer electrolyte fuel cells in which an anistropic gas diffusion layer made of a porous carbon matrix through which carbon particles and poly(vinylidene fluoride) are distributed such that the matrix is homogeneously porous is prepared by casting with a doctor knife onto a carbon substrate a blend of poly(vinylidene fluoride) and carbon black dissolved in a solvent for the poly(vinylidene fluoride) and carbon black to form a layer of film on a carbon substrate resulting in penetration of the mixture into at least a portion of the carbon substrate, coagulating the film in a coagulation liquid that is a non-solvent for the poly(vinylidene fluoride) and carbon black, and removing the coagulation solvent. A catalytic layer comprising a coagulated aqueous ink suspension containing catalytic carbon particles and a thermal plastic polymer is painted onto the surface of the gas diffusion layer.
U.S. Pat. No. 5,935,643 teaches a method for manufacturing an electrode for phosphate-type fuel cells in which an electrocatalyst slurry is coated upon an electrode support which is obtained by waterproofing and sintering carbon paper, dried at high temperature in an inert atmosphere and subjected to a rolling process and then to a sintering process.
U.S. Pat. No. 5,474,857 teaches a solid polymer electrolyte in which the reaction area of the electrode is increased by uniformly dispersing and bonding a solid polymer electrolyte and a catalyst and the ability of gas feeding to the reaction site is improved by adding a fluoropolymer so that the catalyst is not excessively loaded. The electrode, which is provided on at least one side of the solid polymer electrolyte, is formed by coating on one side of a gas diffusible layer a mixed dispersion of a noble metal catalyst, a carbon fine powder and a colloidal dispersion of a solid polymer electrolyte.
U.S. Pat. No. 4,849,253 teaches an electrochemical cell electrode produced by applying a plurality of thin layers of a catalyst material onto a substrate, filtering and compacting the layers between additions, until a desired amount is achieved. The catalyst-bearing substrate is then dried and sintered to form an electrode.
To provide sufficient ionic conductivity within the catalyst layer of the gas diffusion electrode, the platinum/carbon powder catalyst must be intimately intermixed with liquid ionomer electrolyte. Thus, the catalyst layer may be described as a Pt/C/ionomer composite that achieves proton mobility while maintaining adequate electronic conductivity to result in a low contact resistance with the gas diffusion layer. To reduce overall costs, it is desired to maintain Pt metal loading at a minimum.
The proton conducting polymeric membrane is the most unique element of the polymer electrolyte membrane fuel cell. The membrane commonly employed in most recent polymer electrolyte membrane fuel cell technology developments is made of a perfluorocarbon sulfonic acid ionomer such as NAFION(copyright) by DuPont. W. L. Gore, Asahi Chemical and Glass (Japan) produce similar materials as either commercial or developmental products. These membranes exhibit very high long-term chemical stability under both oxidative and reductive environments due to their Teflon-like molecular backbone. This membrane, when wet with water, can serve at the same time as an effective gas separator between fuel and oxidant. If allowed to dry out, gases can pass through the membrane and the fuel cell can be destroyed as hydrogen and oxygen combine in catalytic combustion.
The main step for fabricating MEAs is to catalyze either the gas diffusion electrode or the polymer electrolyte membrane. In either case, an electrode backing is placed on each side of the polymer electrolyte membrane with a catalyst/electrolyte ionomer layer between each gas diffusion electrode and the membrane to form a membrane electrode assembly. Currently, two methods by various developers are used to put the catalyst/electrolyte ionomer layer between the gas diffusion electrode and the polymer electrolyte membrane. One is a direct deposition method; the other is an indirect deposition method.
In the direct deposition method, the catalyst/electrolyte ionomer layer is directly applied to the polymer electrolyte membrane by coating methods, chemical vapor deposition (CVD), physical vapor deposition (PVD), or electrochemical deposition (ECD). The CVD, PVD and ECD methods are not useful in a fuel cell with a gas phase fuel because these methods cannot deposit the electrolyte ionomer with the catalyst particles, as a result of which there is no electrolyte between the catalyst particles in the gas phase. Electrochemical deposition has been used to make MEAs for a direct methanol fuel cell, in which the electrolyte ionomer is not necessary to exist in the catalyst layer because of the liquid phase fuel. In gas phase fuel cells, the catalyst ink can be directly deposited on the polymer electrolyte membrane surface if the membrane does not wrinkle after touching the solvent in the catalyst ink. Coating methods, such as painting, spraying, screen-printing, etc. are generally used to put catalyst/ionomer ink on the membrane surface. These methods create good contact between the catalyst layer and the electrolyte membrane. To maintain good contact in the three phase (gas/electrolyte/catalyst) area, crack-free gas diffusion backing is required to support the catalyst layer. In the fuel cell, the ionic impedance is a main loss in comparison to electrical loss. In other words, the contact between the gas diffusion layer and the catalyst layer is for current collection, that is, electrical connection. The contact between the catalyst layer and the electrolyte membrane is for ionic transportation. As is seen, the direct deposition method reduces the ionic impedance in the fuel cell. The requirement for this method is that the polymer electrolyte membrane must not be sensitive to the ink solvent. A certain clamp force must also be maintained to reduce the electrical resistance between the catalyst layer and the gas diffusion backing.
In indirect deposition methods, the catalyst layer is deposited on a substrate that then decals to the electrolyte membrane or on the gas diffusion electrode that then sandwiches to the electrolyte membrane by hot pressing, hot rolling, or laminating. In one known implementation of the decal method, a layer of catalyst ink is brushed onto a Teflon-coated fiber substrate. After drying, the ink layer with the substrate is hot pressed on a NAFION electrolyte membrane. Although resulting in good contact between the catalyst layer and the electrolyte membrane, this method is limited to producing only small electrodes due to the problem of catalyst releasing from the substrate. In addition, it is very difficult to scale up. A certain clamp force is also required to reduce the electrical resistance between the catalyst layer and the gas diffusion layer.
Catalyst ink deposition on a gas diffusion electrode is another method of producing an MEA. In this method, catalyst ink is deposited onto the gas diffusion electrode which is then either hot-pressed, hot-rolled, or laminated to the polymer electrolyte membrane. This method produces MEAs having good electrical contact between the gas diffusion electrode and the catalyst layer as well as the catalyst layer and the electrolyte membrane. The critical requirement with this method is that the gas diffusion electrode must be crack-free; otherwise the catalyst ink will be lost in the cracks after deposition of the gas diffusion electrode. Consideration must also be given to optimization of the hot-pressing, hot-rolling or laminating force so as to preclude crushing the gas diffusion electrode.
Accordingly, it is an object of this invention to provide a method for producing gas diffusion electrodes and MEAs employing such gas diffusion electrodes that addresses the problems attendant to conventional methods as discussed hereinabove.
These problems are addressed by the method of this invention in which a gas diffusion electrode is produced by mixing a slurry comprising carbon black, at least one alcohol and water with a tetrafluoroethylene (TEFLON(copyright)) emulsion to form a tetrafluoroethylene slurry which, in turn, is applied to a carbon cloth substrate that has not been treated with tetrafluoroethylene, forming a coated carbon cloth. The coated carbon cloth is then heated to a temperature suitable for driving off water, producing a substantially water-free, or dried, coated carbon cloth. The substantially water-free coated carbon cloth is then rolled to substantially eliminate cracks and then heated to a temperature suitable for removing wetting agents from the tetrafluoroethylene emulsion. This water-free and wetting agent-free coated carbon cloth is then cooled, forming a cooled coated carbon cloth. The cooled coated carbon cloth is then rolled to produce the end product gas diffusion elctrode.
In contrast to known methods for producing gas diffusion electrodes which require the impregnation of the carbon cloth with tetrafluoroethylene, the method of this invention does not require such impregnation, or Teflonization, of the carbon cloth. Indeed, impregnation of the carbon cloth in accordance with known methods is undesirable because it results in higher internal resistance of the fuel cell. In addition, the tetrafluoroethylene layer on the carbon cloth has been found to degrade over time. And, as a result of this loss of tetrafluoroethylene coating effectiveness, the hydrophobicity of the backing reduces with time of the fuel cell operation. We have found, however, that even though the carbon cloth is not impregnated with tetrafluoroethylene, the carbon cloth is nevertheless hydrophobic when the sintering temperature reaches 300xc2x0 C. And, because there is no impregnation of the carbon cloth with tetrafluoroethylene, there is no issue of changes in hydrophobicity over time.